For over a century, it has been apparent that fresh serum contains lytic and opsonic factors that are now known as complement. Despite extensive characterization of this system, which includes more than 20 components, its function is still unfolding. The long-term goal of this proposal is to understand how the central component (C3) of this system interacts with the immune system in the acquired immune response and in clearance of encapsulated bacteria. This goal has been divided into three specific aims: (1) construction of a C3 deficient mouse; (2) determine the role of C3 in the humoral immune response; and (3) determine the role of C3 in clearance of encapsulated bacteria. As a novel approach to addressing the long-term objective, mice genetically deficient in complement C3 will be constructed using the technique of homologous recombination in embryonic stem cells. The availability of a totally C3 deficient strain of mouse (C3Def) would be highly important as the interaction between C3 and the well-characterized murine immune system could be examined in vivo for the first time. Individuals deficient in C3 are highly susceptible to repeated pyogenic infections by gram-positive organisms such as Streptococcus pneumonia and Neisseria meningococcus. This study will utilize the C3Def murine model to examine the host's defense against one of these organisms, S. pneumococcus. By clarifying the role of C3 in the humoral response to both T-dependent and T-independent antigens, these results would be important in both identifying those at risk and in designing a possible treatment for infection. For example, if the C3 ligand is found to be critical for bacterial clearance, this ligand might be used as an effective adjuvant in vaccination of immunocompromised children against encapsulated bacteria. In summary, this application proposes to develop for the first time a totally C3 deficient strain of mouse and to use this genetic model to clarify the role of C3 in both the acquired immune response and clearance of bacteria.